Fix non-standard field names in /_recovery API response

Updated field names in SnapshotRecoverySource to use snake_case for consistency across API responses:
- isSearchableSnapshot -> is_searchable_snapshot
- remoteStoreIndexShallowCopy -> remote_store_index_shallow_copy
- sourceRemoteStoreRepository -> source_remote_store_repository
- sourceRemoteTranslogRepository -> source_remote_translog_repository

This change aligns with OpenSearch API naming conventions.

Signed-off-by: Aradhya Mahajan <[email protected]>

server/src/main/java/org/opensearch/cluster/routing/RecoverySource.java

@@ -415,10 +415,10 @@ public void addAdditionalFields(XContentBuilder builder, ToXContent.Params param
                 .field("version", version.toString())
                 .field("index", index.getName())
                 .field("restoreUUID", restoreUUID)
-                .field("isSearchableSnapshot", isSearchableSnapshot)
-                .field("remoteStoreIndexShallowCopy", remoteStoreIndexShallowCopy)
-                .field("sourceRemoteStoreRepository", sourceRemoteStoreRepository)
-                .field("sourceRemoteTranslogRepository", sourceRemoteTranslogRepository);
+                .field("is_searchable_snapshot", isSearchableSnapshot)
+                .field("remote_store_index_shallow_copy", remoteStoreIndexShallowCopy)
+                .field("source_remote_store_repository", sourceRemoteStoreRepository)
+                .field("source_remote_translog_repository", sourceRemoteTranslogRepository);
         }
 
         @Override